There are numerous industrial processes which utilize gas-fired equipment such as furnaces, ovens and driers. Many of them employ multiple stage units requiring multiple burners. Oftentimes they must be fired in a particular sequence. In almost all cases, they must be shut down for a flame failure malfunction in order to avoid the possibility of unwanted combustion or explosion. Associated control systems can be complex or simple, but in all cases they require, as an important input element, sensor circuitry and apparatus for sensing the presence of the flame itself.
Usually, the flame sensor is configured to sense the presence of a pilot flame, to allow the normal sequencing of the equipment when all pilots are sensed as present, and to shut the system down upon failure of any pilot flame. Two types of flame sensing transducers have been developed over the years, and systems are often configured to work with one or the other of such sensors. Each has its respective advantages and disadvantages; in some cases the choice of the type of flame sensor transducer dictates the use of a particular flame sensing interface circuit compatible with it, and thus has broader implications.
One type of flame sensor transducer which has been developed is the flame rod. For present purposes, it is necessary to understand only that the flame rod is a transducer which changes electrical characteristics in the presence of a flame. The transducer is positioned such that it will be in the path of a pilot flame when present. With no flame present, a relatively high alternating voltage coupled to the flame rod will be passed through as an alternating voltage. With flame present, the flame rod will begin to act as a rectifier, with peaks of one polarity getting larger and peaks of the other polarity becoming smaller.
The other type of commonly used transducer is the ultraviolet sensor. It typically operates on a relatively high voltage DC supply, and has an ultraviolet receptor aimed at the pilot flame. The flickering of the pilot flame will cause the output of the ultraviolet sensor to vary, producing an electrical signal which has a ripple component caused by the flicker of the pilot flame. It follows that with no pilot present there will be no ripple and thus a constant DC output.
It will be appreciated that these two types of sensors require separate types of power supplies and separate kinds of interfacing electronic circuitry in order to take advantage of the characteristics of each transducer type. In addition, most flame sensors, at least of the standalone type, provide only a failure indication, in other words, they are bi-state devices, providing one type of signal in the presence of a flame, and another type of signal after a flame failure is detected. Very often, a set of relay driven switch contacts, sometimes driven by an SCR, serve as an output device, with a transition from one state to the other signaling a transition from flame-on to flame failure.
Control systems of reasonable sophistication have been developed to operate large complex furnace or oven systems, and they usually provide forms of sequencing and safety control. Such systems typically require flame sensors as input devices, and can utilize the switch closure feature of typical flame sensors to perform that portion of their function.
Flame sensors can also sometimes be used in standalone fashion, without the need for comprehensive control systems. Prior art flame sensors can be adapted to this use but introduce complexities, such as the need for a complementary power supply, multiple modules for multiple flames, and different kinds of modules for ultraviolet or flame rod operation. It is not unusual for a standalone flame sensor to be just that--a standalone flame sensor. Suppliers of complex systems and standalone flame sensors often utilize different flame sensors for the complex control system. The prior art has attempted to utilize certain standardized modules for multiple purposes. For example, it appears that efforts have been made to utilize a flame sensor module compatible with both flame rods and ultraviolet transducers. However, it is understood that such a device provides only a flame/no-flame indication, and does not provide any additional information on the quality of the sensed flame.